The field of this invention is optical communications systems, and more particularly systems of this type which employ optical fibers which are used to transmit intelligence by means of modulated light beams or waves. The intelligence transmitted may be either data or voice. The use of digital transmission utilizing time division multiplex techniques facilitates the transmission of both voice and data signals. In such a system the voice or any other analog signal to be transmitted is sampled and transmitted as pulse amplitude modulation or coded into a binary number proportional to the amplitude thereof (pulse code modulation). The system of the present invention utilizes a time division multiplex technique known as Time Division Multiple Access (TDMA) in which each of the multiple users transmits on the same wavelength in sequence in a different time slot for pulse transmission and the fiber optics of all users are interconnected by means of a star type coupler which receives each user's transmission and re-transmits all received signals to all users. Such a system has the capability of full duplex transmission wherein any pair of connected users can carry on two way communication simultaneously with all other user pairs. To accomplish this each user transmits on his time slot and listens or receives on the other user's time slot.
The advantages of fiber optic transmission compared to wire are well known. For example, the reduced dispersion of coherent light compared to lower frequency incoherent waves requires fewer repeaters to re-shape the square waves used for digital transmission. Also, fiber optics are not susceptible to electrical or magnetic interference such as is often caused by magnetic storms or lightning and optical cables are more difficult to tap than electrical wires or cables. This latter feature makes fiber optics advantageous for military tactical operations.
Prior art optical communications systems of the type mentioned above have used passive star couplers to interconnect the users. These passive star couplers comprise merely a means to receive transmissions from all connected users and distribute the received transmissions to all users. Two known types of couplers for accomplishing this connection are transmissive couplers in which all incoming optical fibers are applied to one end of a mixing rod and all of the output fibers are connected to the other end thereof. Another type of coupler for this purpose is the reflective coupler which comprises a rod with both the input and output fibers connected to one end thereof and a mirror at the opposite end. These passive couplers split the energy of all incoming light signals between all of the connected users and in addition attenuate the incoming signals so that for large systems with numerous users the light signal strength available for retransmission is small, thus restricting system range.